Originally Posted by ependydad
Interesting. I'm having the same issue, but only when a toilet is flushed and the main A/C is running.
When the toilet is flushed (flapper being held open) and anything (range hood, exhaust fan, etc.) is pushing air out of the trailer, air to replace the exhausted air has to come from somewhere.
And that somewhere is down the vent stack, through the black tank, and up through the open flush flapper.
Moral of the story: Don't turn on the bathroom exhaust fan until after you've completed flushing.